This invention relates to solar power systems, and more particularly to solar power systems which provide useful power for immediate consumption and also provide energy which may be stored for later use. The characteristics of the components of the systems enable a photovoltaic array to operate near its peak power point over a wide range of solar insolation levels without an active power control device.
Photovoltaic arrays convert the electromagnetic energy of the sun into electrical energy. The electricity may be consumed immediately, or it may be converted into another form of energy and stored for later use. Often, the electricity is used to charge batteries or power electrical loads, including water electrolysis units. A water electrolysis unit uses electricity to convert water into hydrogen and oxygen gas. The hydrogen may be stored and used at a later time to fuel a number of other energy generating devices, including fuel cells.
The use of solar arrays to power electrolysis equipment is described in U.S. Pat. No. 4,160,816. U.S. Pat. No. 4,021,323 teaches use of the current produced by exposure of photovoltaic cells to light to produce an electrochemical reaction in an electrolyte in contact with the photovoltaic sources. Exemplary is the production of hydrogen from an aqueous solution of hydrogen iodide. However, electrical energy is not produced for immediate consumption outside of the electrolysis unit.
A photovoltaic array is essentially a constant current source at a given insolation level, particularly when operated at voltages less than that which produces the maximum power output of the array. The constant current is essentially a function of the insolation level.
The voltage output of a photovoltaic array may range from zero under short circuit conditions to its peak (an array design parameter) under a no load condition. The insolation level will have some effect on the voltage at which a photovoltaic array develops its peak power. The insolation level has a minor effect on the no load voltage, but the short circuit current is directly proportional to the isolation level.
The power output potential of a photovoltaic array is a function of the intensity of the solar radiation or insolation level. The actual power output is a function of the voltage developed across the load and the current drawn by the load.
Photovoltaic arrays may be operated over a range of voltage-current levels or power points. For any particular insolation level, however, there is a power point at which a photovoltaic array operates at its maximum or peak power. A locus of such peak power points may be identified for various insolation levels.
The array load may consist of one or more devices and may have a resistance which is either fixed, varies as a function of the demand or varies as a function of the voltage applied. A resistive heating element is an example of a fixed resistance device which may convert electrical energy into heat for applications such as water heating, cooking or any other process which requires heat. A D.C. to A.C. inverter is an example of a load which can vary its input resistance (generally termed impedance) as a function of demand. An inverter converts direct current electrical energy into alternating current electrical energy to provide power to A.C. device(s). The A.C. device(s) may be turned on and off randomly according to the demands of the user. A water electrolysis unit is an example of a load which varies its impedance as a function of the voltage applied.
An electrolysis unit may be designed which has a voltage-current characteristic similar to the locus of peak power points of a photovoltaic array, as suggested in a Final Report to the California Institute of Technology Jet Propulsion Laboratory pursuant to Contract No. 955492, June, 1980, by R. W. Foster, R. R. Tison, W. J. D. Escher and J. A. Hanson. Perfect assimilation over a wide range of power levels is difficult to attain, however, and the voltage-current characteristic, or impedance (resistance), of the electrolysis unit changes as it is used. The resistance of the electrolysis unit decreases as cell temperatures increase. An increase in the amount of hydrogen dissolved in the cathode increases the unit's resistance. Other factors, such as electrode activity and carbonate formation, also affect the voltage-current characteristic of the unit. In addition, the photovoltaic array's peak power points will shift somewhat as a function of the array's operating temperature. Thus, the electrolysis unit may not always operate near the peak power points of the photovoltaic array.
The use of an active power control device has been suggested as a means of maintaining the array at its peak power points in a paper presented at the Annual Meeting of the International Solar Energy Society, Los Angeles, Calif., 1975, by E. N. Costogue and R. K. Yasui, in another paper presented at the 11th Intersociety Energy Conference, State Line, Nev., 1976, by K. E. Cox, and in yet another paper presented at the 3rd World Hydrogen Energy Conference, Tokyo, Japan, June, 1980, by D. Esteve, C. Ganibal, D. Steinmetz and A. Vialaron. The control device suggested in these papers is essentially a DC-DC converter connected in series between the photovoltaic array and the electrolysis unit. A power sensing circuit varies the input impedance of the converter so that the photovoltaic array operates at its peak power point at all times. These papers suggest that the power output of solar systems may be maximized by such an active power control device.
Cascading electrolysis cells to synthesize a desired impedance by using an active switching device which switches electrolysis cells in and out of the system is a means of maximizing the power output of the array. An active array switch device that switches portions of the photovoltaic array in and out of the system may also be used. However, none of the prior art known to the inventor suggests means of operating the photovoltaic array near its peak operating point and compensating for changes in the impedance of the electrolysis unit and the power available from the photovoltaic array without the use of an active power control device or an active switching device of some sort.
An active power control device compensates for changes in impedance by matching the impedance of the electrolysis unit to that of the photovoltaic array or vice versa or both. However, while the active power control device may maintain the operating efficiency of the photovoltaic array at a fairly high level, i.e., 95 percent or greater, the control device itself requires 5 or 10 percent of the power produced by the photovoltaic array. Thus, the maximum power which may be transferred and used is about 90 to 95 percent of the maximum power available from the photovoltaic array. Thus, there is a need for a solar power system which provides more than 90 to 95 percent of the maximum power available from the photovoltaic array for immediate consumption and energy storage, for a more efficient system.
The active power control device also increase the cost and complexity of the system. The increased complexity may result in decreased reliability. Thus, there is a need for a solar power control system which eliminates an active power control device to reduce costs and the complexity of the system. Reducing complexity increases reliability.